The present application claims priority from German Patent Application No. 100 51 420.0, filed Oct. 17, 2000.
The invention relates to a cylinder block of an axial piston compressor, in particular for use in a vehicle air conditioner employing CO2 as coolant.
The air conditioners installed in motor vehicles employ coolant compressors of various constructions. In recent years, however, so-called axial piston compressors have become predominant. FIG. 4 shows this kind of axial piston compressor according to the state of the art, in diagrammatic longitudinal section. This is a so-called swash-plate compressor to be used with R134a as the coolant. A compressor shaft 1 is driven by way of a pulley associated with a magnetic coupling 2. A swash plate 3 is coupled to the drive shaft 1. The swash plate 3 is fixedly attached to the drive shaft 1 and rotates therewith. Various forms of linkage are known for mounting and centering the swash plate 3 on the drive shaft 1 in such a way that although the plate cannot rotate with respect to the shaft, it is possible for the axis 3a of the swash plate 3 to be tilted relative to the axis 1a of the drive shaft 1. The tilt angle of the swash plate 3 as a rule is limited to a minimal (3b) and a maximal (3c) value by two stopping devices. Customarily one or two guide pins 4 are needed so that the tilting movement can be carried out in a specified manner, with no risk of the plate becoming jammed during the adjustment. The means of limiting the tilting movement, i.e. the limit stops, can also be integrated into the region of the guide pins 4. In the example shown in FIG. 4, the guide pin is fixedly attached to the swash plate 3 and incorporates a bearing 5a that is movable with respect to a stopping plate 5 and that is responsible for centering the guide pin (see reference numeral 4a in FIG. 4). The stopping plate 5, which is likewise attached to the shaft, is supported by way of an axial bearing 6.
The swash plate is not only adjustable regarding its angle with respect to the drive shaft 1, but can also be shifted axially along the drive shaft 1. This shifting is necessary so that the top-dead-center point of the associated piston 8 can be maintained despite differences in the plate angle. Ordinarily limit stops are provided that prevent the swash plate 3 from being displaced beyond an upper seating 3d and a lower seating 3e on the drive shaft 1. The displacement mechanism is pretensioned to a specified degree by a compression spring 7. The stroke of the piston is determined by the tilt angle of the swash plate 3. When this angle is as large as possible, the stroke is maximal; a minimal tilt angle results in a minimal piston stroke.
The pistons 8 in the embodiment illustrated here are connected to the swash plate 3 by way of hemispherical linkage elements 9, 10. To absorb the tensile-pressure load, above a piston one linkage element 10 is disposed on the lower bearing surface 3gof the swash plate 3 and another linkage element 9 is disposed on its upper bearing surface 3f. By way of their flat surfaces 9a, 10a, the linkage elements run over complementary bearing surfaces of the swash plate 3 at the full circumferential velocity with a radial movement superimposed, with the result that the path of the linkage elements on the swash plate is elliptical. The convex upper surfaces 9b, 10b of the linkage elements are seated in indentations in the piston 8 that have a complementary, hemispherical form; while the compressor is in operation, there is a comparatively small amount of relative movement here. The axial piston compressors described above comprise several pistons distributed about their periphery, customarily three to eight pistons.
FIG. 9 shows a side view of a piston 8 such as is employed in an axial piston compressor like that shown in FIG. 4. The piston 8 consists of two sections, namely a piston shaft 8a and a piston neck 8b. The term xe2x80x9cshaftxe2x80x9d is used here to designate the part or section of the piston that is disposed within an associated cylinder bore which guides its back-and-forth it movement. The piston neck 8b, which customarily comprises a U-shaped cavity 8c, encloses the above-mentioned swash plate 3 and, in combination with the linkage elements 9, 10, serves to transmit the forces from the swash plate 3 to the piston shaft 8a. The back surface of the piston neck 8b is in contact with the inner surface of the drive-mechanism housing, in such a way that as the piston 8 moves back and forth, it is prevented from rotating within the associated cylinder bore.
FIG. 7 shows a specific way in which this rotational stability is accomplished. Supplementary information is provided in the document EP 0 740 076 A2. In order to limit the rotation of the piston 8 about its long axis, on the side of the piston neck that faces the drive-mechanism housing a convex surface 72 is disposed. Opposite to and spaced somewhat away from this surface is a concave surface in the housing 71. The radius R1 of the convex surface 72 is larger than the radius Rp of the cylindrical outer surface of the piston shaft, but smaller than the radius R2 of the concave inner surface in the housing 71. The contact between the convex surface 72 and the concave surface of the housing 71 limits the extent to which the piston 8 can rotate about its long axis. The dashed line shows that if the piston 8 rotates, only one edge 74 of the convex surface 72 touches the concave inner surface of the housing 71. To reduce friction and avoid wear and tear, it is advisable to treat the surfaces appropriately.
In FIG. 8 an alternative means of guiding the piston longitudinally is illustrated. This alternative construction to prevent unintended rotation of the piston 8 comprises a ridge 82 disposed along the piston shaft, which engages with a corresponding groove 83 in the face of the cylinder bore 81.
The translational movement of the piston 8 in the associated cylinder bore requires the dimensions, shape, position and surface properties of the parts and/or surfaces that correspond to one another to be very close to specifications.
FIGS. 4, 7, 8 and 9 document the classical state of the art insofar as it pertains to the axial piston compressor for a vehicle air conditioner. When R134a is used as the coolant, the piston diameter is about 30 mm. Because CO2 is a distinctly higher-performance coolant, compressers employing CO2 can have a considerably smaller stroke volume. On the other hand, it is necessary to cope with comparatively large pressure differences. That is, when CO2 is used as the coolant, the pressures exerted on the piston 8 are considerably greater. To compensate for these higher pressures, CO2 compressors are provided with pistons of considerably smaller diameter, e.g. about 16 mm.
However, as is illustrated in FIG. 5, when the shaft of a piston has such a small diameter, its neck projects further outward; that is, it is displaced to the side with respect to the long axis of the piston. In the embodiment shown in FIG. 5 the pistons 8 are driven by a wobble plate 29. The wobble plate 29 rests against a swash plate 33 by way of antifriction bearings 30, 28 and 24. By means of an internally threaded fixation disk 31, the wobble plate is fixed so that its axis coincides with that of the swash plate. The pistons are coupled to the wobble plate by way of linkage bearings 27 and 25 and by a set screw 26. The piston 8 consists of a shaft 8a and a neck 8b. These are separated by a transitional region 8c, which connects the shaft to the neck. In the case of an R134a compressor this transitional region is not necessary, because (as shown in FIG. 9) the neck of the piston has a diameter no greater than that of the associated shaft. The pistons 8 shown in FIG. 5 are guided axially in their back-and-forth movement by bores within a cylinder block 32. If the pistons 8 are provided with piston rings, the bearing surface of the cylinder bores either is coated with a layer of particularly wear-resistant material, or there are set into it bushings 26 made of a correspondingly resistant material. As can be seen in FIG. 5, the bushings 26 at the drive-mechanism end of the cylinder are flush with the cylinder block. Accordingly, the transitional region 8c between the shaft and neck of the piston is always outside the bearing surface of the cylinder.
To put it simply, the forces acting on the piston 8 are a transverse force FQ (introduced by the drive mechanism) and transverse forces FA and FB, which are also applied to the cylinder bushing 26 (see FIG. 5). One of the factors determining the magnitude of the reactive forces FA and FB is the lever-arm ratio B/(B-A).
These transverse forces are plotted in FIG. 6 for one operating point. The lever-arm ratio is also plotted in this graph. The relationships between force and angle shown there must be regarded as qualitative, because they can be shifted or altered in magnitude to a considerable extent at different operating points.
When the drive shaft 1 for the axial piston compressor is at the angular position 0xc2x0, in the embodiment illustrated here, the piston 8 has reached its top dead center (OT), the position in which the coolant gas is being expelled from the compression space. As the angle increases, the piston moves towards its bottom dead center (UT), which it reaches when the drive shaft is in the 180xc2x0 position. At the bottom dead center the pressure in the compression space falls to the suction pressure, at which point traction forces operate. During the further rotation of the drive shaft 1, beyond 180xc2x0, the piston is returned to top dead center. In the process, the coolant gas is compressed and subsequently expelled from the compression space. In FIG. 5 the transverse forces are represented as vectors, as mentioned above. FIG. 5 shows the piston at top dead center, at which point the angular position of the drive shaft 1 is 0xc2x0. The transverse forces FQ are of relatively large amplitude in this position and are directed towards the central axis of the compressor. The transverse force FQ introduces the reactive forces FA and FB, also shown as vectors in this xe2x80x9csnapshotxe2x80x9d, as a result of a bending moment at the piston 8. During a complete piston movement (OT-UT-OT) the transverse forces make one complete circuit about the piston axis.
The critical region for the drive shaft 1 is in the range of angular positions from 250xc2x0 to 360xc2x0, because in this range the transverse forces are relatively large. This applies in particular to a drive-shaft position of 270xc2x0, in which the piston 8 is still pulled relatively far out of the cylinder liner 26. This signifies a particularly large reactive force FA. Accordingly, in this angular position (270xc2x0) the pistons 8 are especially exposed to wear. The diagram according to FIG. 6 also makes clear that the pistons, for instance in the angular position 180xc2x0 (UT), cannot be braced against the wall of the housing because then traction forces would act.
Finally, it is evident from FIG. 5 that if the introduced transverse force FQ encountered a support directly in the region where it is applied, a bending moment in the piston would be prevented. This would bring enormous advantages with respect to reducing wear and tear.
Accordingly, the objective of the present invention is to create a cylinder block for an axial piston compressor, the cylinder faces of which ensure considerably improved support of transverse forces, in order by this means to increase the working life of the axial piston compressor.
The central idea in the present invention is thus that the edge of the opening of the cylinder face on the drive-mechanism side comprises at least one recess with which a transitional region disposed between the shaft and the neck of the piston can engage, so that the cylinder face effective for the piston is enlarged or lengthened. As a result, the lateral support of the piston against the associated bearing surface in the cylinder bore is considerably improved, with the consequence that there is less wear and the working life of the axial piston compressor is prolonged.
Moreover, in the present invention the fundamental rule applies that the recess in accordance with the invention is in each case disposed at the site that is under the least load. To assist understanding of this basic idea, reference is made to FIGS. 10a to 10c. These drawings show the paths of the transverse forces around the piston and the length of its contact region in the course of a stroke; these two parameters determine the amount of wear and hence also the preferred position of the lengthened cylinder face. At top dead center OT the length of the contact region is maximal, whereas at bottom dead center UT it is minimal. Now if the revolving transverse force is projected onto the contact length, as shown in FIG. 10b, the sequence of priorities for a lengthened support surface for the piston, i.e. a correspondingly lengthened cylinder face, is as follows: D, A, B. C. That is, in position C the load is least, and hence this is the preferred position for construction of a recess in accordance with the invention. The position C is radially external to the longitudinal middle axis of the axial piston compressor and of the swash or wobble plate associated with the piston 8; that is, it is next to the inside of the drive-mechanism housing. At this juncture it should once again be mentioned that FIG. 10a leaves open the direction in which the swash or wobble plate rotates. Depending on the direction of rotation of the swash or wobble plate, of course, there will be a mirror-image reversal of the paths about the X (horizontal) axis shown in FIGS. 10b and 10c. 
The first alternative is characterized by the fact that the surface of the cylinder bore is worked; i.e., there is no separate liner or similar component. Accordingly, in this embodiment the above-mentioned recess must be formed within the cylinder bore, at the edge of the opening on the drive-mechanism side.
The second alternative is characterized by the insertion into the at least one cylinder bore of a liner that projects from the cylinder bore on the drive-mechanism side and in this region is provided with the above-mentioned recess.
The third alternative is characterized in that, again, a liner is inserted into the at least one cylinder bore, but in this case its edge is flush with the opening of the cylinder bore on the drive-mechanism side. In this case the recess in accordance with the invention must be formed in both the cylinder bore and the associated liner, at the edge of the opening on the drive-mechanism side.
As can be discerned from the above explanations in combination with FIGS. 10a to 10c, the recess in accordance with the invention is situated in a region in which the transverse forces acting on the piston are weakest. This region is ordinarily in the position on the side of the piston opposite the coupling to the drive mechanism, i.e. the side next to the wall of the drive-mechanism housing.
Preferably the neck of the piston is also laterally supported against the drive-mechanism housing. The lateral support here is brought about by a guide pin, a feather key or a similar means of longitudinal guidance. Models for this are available in the state of the art.
Preferably the edge of the opening of the cylinder face on the drive-mechanism side is beveled or rounded, to facilitate installation of the piston and reduce wear on the piston as it moves past the edge.
The recess in accordance with the invention preferably extends over an angular region of about 80xc2x0 to about 160xc2x0, in particular about 120xc2x0.